A problem for many organizations is the low continuity of volunteers. Diverse theoretical models have been developed to explain sustained volunteerism, but most of these models have focused on the study of welfare volunteerism and not have verified whether their results could be generalized to other types of volunteerism. In this study, we propose a basic model to explain sustained volunteerism in any type of volunteerism. The aim of this study is to examine this basic model empirically in a sample of socio-assistential volunteers and in a sample of environmental volunteers. For this purpose, 290 volunteers completed a questionnaire and subsequently, two telephone follow-ups were carried out to determine whether the volunteers remained in the NGO one year later. The results show that the basic model offers an acceptable explanation of sustained volunteerism in the two types of volunteer samples studied, but it accounted for a low percentage of variance of the duration of service in both samples. Various relevant implications for the practical management of volunteerism in organizations are derived from theses findings.